From what I've read from buyers so far I'd be cautious.
Problems with AF and the OS, taking often up to 2 seconds to work! I'd find that really annoying. There's also some problems with 40D's with the OS, Sigma has already had to issue a firmware update for some units. Some have said it's sharp, others it's soft...They're probably all right.

Sigma are notorious for sample variations.
I'd say wait a few months if you can, see if these problems are sorted. There are a few sites on the web with sample images. Some I've seen are terrible, others are fine. I'm always cautious of bad images without knowing how the shots were taken. Just do a Google search or go to Fredmiranda.com, there's plenty of samples there.
